1. Field of the Invention
This invention relates to voltage reference circuits, and more particularly to voltage references in which the user can select among a number of different output reference levels using only a single control pin.
2. Description of the Related Art
Voltage reference circuits are designed to produce a known fixed reference voltage at an output terminal. It is often desirable to allow the user to select among different fixed regulated voltage levels, or alternately to set his or her own regulated output at a level different from the levels designed into the circuit. A major obstacle to this type of capability is that only a very limited number of pins are typically available on an integrated circuit (IC) chip. While circuits could easily be designed with multiple input pins used to select among different output voltage levels, with each pin dedicated to a particular output, more than one input control pin may not be available for this purpose.
In U.S. Pat. No. 4,797,569 a single pin is used to select between a five volt reference output and a mode in which the user can establish the output level by connecting a user-selected external resistor circuit. An operational amplifier within the circuit includes a feedback circuit that establishes the 5 volt reference output. A switch is provided within the feedback circuit, and another switch is connected between a control terminal and the amplifier. When the user supplied voltage applied to the control terminal is less than a preset threshold level, as determined by a voltage comparator within the regulator circuit, the feedback switch is closed and the switch between the control terminal and the amplifier is opened; this allows the amplifier to establish the voltage at the output terminal at the predetermined 5 volt level. The switching pattern is reversed when the user applies a voltage above the threshold level to the control terminal. In this second mode a feedback circuit for the amplifier is completed by connecting a resistor circuit between the control and output terminals; the particular resistor values selected by the user determine the output reference voltage as the ratio of two external resistors.
While this patent represents an improvement in regulated voltage circuits, it is limited to a single internally generated output voltage level. It would be desirable to allow the user to select among multiple fixed internally generated voltages, without having to use more pins for this feature, while preserving a capability for the user to set his or her own output level.